[Therion] GIS model output - open multipatch shp

Hi, I use the GIS model output to calculate the volume of a cave. To do it the multipatch have to be closed. The multipatch shp that therion exports is never close, even when everything is ok. To fix it, I use a tool in arc scene to close the multipatch shp, but it always adds an extra volume to the cave, sometimes it's significant. So my question is why the multipatch isn't close? Can it be fixed? Omri